The present invention relates to a spindle assembly for magnetic disks and, more particularly, to a spindle assembly of the type supported at both ends thereof and designed to permit an ideal location of bearings.
The current trend for greater memory capacity of magnetic disk requires higher performance of the spindle for driving the disk, particularly higher precision of control of rotation of the spindle. The heretofore known spindles for magnetic disks can be broadly sorted into two types: namely, cantilever type and the type in which the spindle is supported at its both ends. In the cantilever type construction, a pair of bearings are mounted in a single housing, and a hub for fixing the disk is fixed to either one of the shaft ends. In this type of construction, a pair of bearing boxes for receiving respective bearings are formed simultaneously in a single housing, so that it is comparatively easy to attain the required degree of axial alignment or concentricity between two bearing boxes. In addition, since the housing is a single member, the spindle assembly can be completed to the final state, offering various advantages such as easiness of examination of characteristics such as the precision of rotation, high producibility and so forth. Unfortunately, however, this type of construction suffers from a disadvantage in that the rotation precision is undesirably limited because the hub is fixed at a position outside the region between two bearings, i.e., due to the arrangement known as "overhang" type structure. The cantilever type construction, therefore, is suited to small-sized devices in which any slight degradation in the spindle precision does not matter substantially.
In recent years, in order to meet the demand for higher precision of the spindle rotation, the second type construction, i.e., the type in which the spindle is supported at both ends, is becoming major rather than the cantilevered type construction. This type of construction will be referred to as "both-end type" construction, hereinunder.
The both-end support type construction ensures a high rigidity of the structure and, hence, is superior. This type of construction, however, encounters a problem concerning difficulty in setting a pair of bearings on a base at a high precision after assembly of the disk, resulting in a low producibility.
In this type of supporting structure, the housing for supporting the pair of bearings is composed of two halves, e.g., left and right half parts or upper and lower half parts. This arrangement permits the hub for fixing the disk to be positioned at a good balance between both bearings such as to ensure a high precision of rotation. In fact, the precision of spindle rotation in this type of construction is 10 or more times as high as that provided by the cantilevered type construction.
The drawbacks of this type of construction will be explained with specific reference to FIG. 2 which is a sectional view of a typical conventional construction of spindle assembly of both-end support type. The process of assembly is as follows. As a first step, a shaft 21 to which a hub 26 is fixed by, for example, shrink fit is inserted into a bore formed in a lower housing 15 from the lower side. Then, various parts such as a pre-loading spring 27, lower bearing 28, collar 29, motor rotor 32 and a washer 30 are fitted around the shaft 21 and inserted into the bore in the lower housing. These parts are fastened by a fastening screw 31. Then, after a motor stator 34 is fitted in the bore formed in the lower housing, a cover 33 is secured such as to close the bore. Subsequently, magnetic disks 3 are secured to the spindle through a hub 26, with spacers 25 placed between adjacent disks. Then, the disks are fixed by means of screws 20 and a disk clamp 24.
Subsequently, an upper housing 16 is jointed to the lower housing 15 by screws 22 while being located by a pin 23. Finally, an upper bearing 19 and a washer 18 are fitted around the upper portion of the shaft 21 and fastened by means of a screw 17, thus completing the assembly.
The most significant requisite for enjoying the performance of this spindle assembly is to attain a high degree of concentricity between the upper and lower bearings. Namely, it is highly desirable that the assembly is completed in such a manner as to provide a high degree of coincidence between the center of the bearing box in the upper housing 16 and the center of the bearing box in the lower housing 15.
As will be understood from the foregoing explanation taken in conjunction with FIG. 2, since the upper housing 16 is located with respect to the lower housing 15 by means of the pin 23, the degree of concentricity between both bearing boxes is affected by factors such as the precision of the diameter of bore for receiving the pin 23, precision of positioning of the pin 23, and so forth. For this reason, it has been quite difficult to attain a high degree of concentricity less than 0.06 mm between both bearing boxes. This value of precision is quite unsatisfactory in the case of the both-end support type construction, considering that a high precision on the order of 0.01 mm is attainable even with the cantilever type construction.
Thus, with the known arrangement, it has been impossible to make full use of the advantage of the both-end support type construction, i.e., the high degree of precision inherent in this type of construction.